Loop peeling

* module/language/cps/peel-loops.scm: New pass.  Only enabled if the
  loop has one successor.

* module/language/cps/optimize.scm: Peel instead of doing LICM on
  higher-order CPS, then LICM on first-order CPS.

* module/Makefile.am: Wire up new pass.

module/Makefile.am

@@ -131,6 +131,7 @@ CPS_LANG_SOURCES =						\
   language/cps/effects-analysis.scm				\
   language/cps/elide-values.scm					\
   language/cps/licm.scm						\
+  language/cps/peel-loops.scm					\
   language/cps/primitives.scm					\
   language/cps/prune-bailouts.scm				\
   language/cps/prune-top-level-scopes.scm			\

module/language/cps/optimize.scm

@@ -30,6 +30,7 @@
   #:use-module (language cps dce)
   #:use-module (language cps elide-values)
   #:use-module (language cps licm)
+  #:use-module (language cps peel-loops)
   #:use-module (language cps prune-top-level-scopes)
   #:use-module (language cps prune-bailouts)
   #:use-module (language cps rotate-loops)
@@ -82,11 +83,6 @@
 ;;  * Abort contification: turning abort primcalls into continuation
 ;;    calls, and eliding prompts if possible.
 ;;
-;;  * Loop peeling.  Unrolls the first round through a loop if the
-;;    loop has effects that CSE can work on.  Requires effects
-;;    analysis.  When run before CSE, loop peeling is the equivalent
-;;    of loop-invariant code motion (LICM).
-;;
 (define-optimizer optimize-higher-order-cps
   (split-rec #:split-rec? #t)
   (eliminate-dead-code #:eliminate-dead-code? #t)
@@ -97,7 +93,7 @@
   (specialize-primcalls #:specialize-primcalls? #t)
   (elide-values #:elide-values? #t)
   (prune-bailouts #:prune-bailouts? #t)
-  (hoist-loop-invariant-code #:licm? #t)
+  (peel-loops #:peel-loops? #t)
   (eliminate-common-subexpressions #:cse? #t)
   (type-fold #:type-fold? #t)
   (resolve-self-references #:resolve-self-references? #t)
@@ -105,6 +101,8 @@
   (simplify #:simplify? #t))
 
 (define-optimizer optimize-first-order-cps
+  (hoist-loop-invariant-code #:licm? #t)
+  ;; FIXME: CSE here to eliminate duplicate free-ref terms.
   (eliminate-dead-code #:eliminate-dead-code? #t)
   (rotate-loops #:rotate-loops? #t)
   (simplify #:simplify? #t))

module/language/cps/peel-loops.scm

@@ -0,0 +1,287 @@
+;;; Continuation-passing style (CPS) intermediate language (IL)
+
+;; Copyright (C) 2013, 2014, 2015 Free Software Foundation, Inc.
+
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+;;; Commentary:
+;;;
+;;; Loop peeling "peels off" one iteration of a loop.  When followed by
+;;; common subexpression elimination, it has the effect of moving terms
+;;; to the first peeled iteration, leaving the loop body with fewer
+;;; terms.
+;;;
+;;; Loop peeling is complementary to loop-invariant code motion (LICM).
+;;; LICM will hoist invariant terms that have no side effects, like
+;;; $const, even if they are in branches that are not always taken.
+;;; However LICM won't hoist expressions that might have side effects if
+;;; it can't prove that they are reachable on every iteration.  Peeling
+;;; on the other hand arranges for the body to be dominated by one loop
+;;; iteration, so any effect that is reachable on one full iteration can
+;;; be hoisted and eliminated, which is a big boon when we consider
+;;; &type-check effects.  For example:
+;;;
+;;;    x = cached-toplevel-box map
+;;;    y = box-ref x
+;;;    z = cached-toplevel-box foo
+;;;    w = box-ref z
+;;;    ...
+;;;
+;;; In this example, LICM could hoist X, possibly Y as well if it can
+;;; prove that the body doesn't write to variables, but it won't hoist
+;;; Z.  In contrast, peeling + CSE will allow Z to be hoisted.
+;;;
+;;; Peeling does cause code growth.  If this becomes a problem we will
+;;; need to apply heuristics to limit its applicability.
+;;;
+;;; Implementation-wise, things are complicated by values flowing out of
+;;; the loop.  We actually perform this transformation only on loops
+;;; that have a single exit continuation, so that we define values
+;;; flowing out in one place.  We rename the loop variables in two
+;;; places internally: one for the peeled iteration, and another for
+;;; the body.  The loop variables' original names are then bound in a
+;;; join continuation for use by successor code.
+;;;
+;;; Code:
+
+(define-module (language cps peel-loops)
+  #:use-module (ice-9 match)
+  #:use-module ((srfi srfi-1) #:select (fold))
+  #:use-module (language cps)
+  #:use-module (language cps utils)
+  #:use-module (language cps intmap)
+  #:use-module (language cps intset)
+  #:export (peel-loops))
+
+(define (intset-map f set)
+  (persistent-intmap
+   (intset-fold (lambda (i out) (intmap-add! out i (f i))) set empty-intmap)))
+
+(define (loop-successors scc succs)
+  (intset-subtract (intset-fold (lambda (label exits)
+                                  (intset-union exits (intmap-ref succs label)))
+                                scc empty-intset)
+                   scc))
+
+(define (find-exits scc succs)
+  (intset-fold (lambda (label exits)
+                 (if (eq? empty-intset
+                          (intset-subtract (intmap-ref succs label) scc))
+                     exits
+                     (intset-add exits label)))
+               scc
+               empty-intset))
+
+(define (find-entry scc preds)
+  (trivial-intset (find-exits scc preds)))
+
+(define (list->intset vars)
+  (persistent-intset
+   (fold1 (lambda (var set) (intset-add! set var)) vars empty-intset)))
+
+(define (compute-live-variables cps entry body succs)
+  (let* ((succs (intset-map (lambda (label)
+                              (intset-intersect (intmap-ref succs label) body))
+                            body))
+         (init (intset-map (lambda (label) #f) body))
+         (kill (intset-map (lambda (label) #f) body))
+         (gen  (intset-map (lambda (label)
+                             (match (intmap-ref cps label)
+                               (($ $kargs names vars) (list->intset vars))
+                               (_ empty-intset)))
+                           body))
+         (in (intmap-replace init entry (intmap-ref gen entry)))
+         (out init))
+    (define (subtract in kill) (or in empty-intset))
+    (define (add in gen) (if in (intset-union in gen) gen))
+    (define (meet in out) (if in (intset-intersect in out) out))
+    (call-with-values (lambda ()
+                        (solve-flow-equations succs in out kill gen
+                                              subtract add meet
+                                              (intset entry)))
+      (lambda (in out)
+        out))))
+
+(define (compute-out-vars cps entry body succs exit)
+  (let ((live (compute-live-variables cps entry body succs)))
+    (intset-fold-right
+     cons
+     (intmap-fold (lambda (label succs live-out)
+                    (if (intset-ref succs exit)
+                        (if live-out
+                            (intset-intersect live-out (intmap-ref live label))
+                            (intmap-ref live label))
+                        live-out))
+                  succs #f)
+     '())))
+
+(define (rename-cont cont fresh-labels fresh-vars)
+  (define (rename-label label)
+    (intmap-ref fresh-labels label (lambda (label) label)))
+  (define (rename-var var)
+    (intmap-ref fresh-vars var (lambda (var) var)))
+  (define (rename-exp exp)
+    (rewrite-exp exp
+      ((or ($ $const) ($ $prim) ($ $closure) ($ $rec ())) ,exp)
+      (($ $values args)
+       ($values ,(map rename-var args)))
+      (($ $call proc args)
+       ($call (rename-var proc) ,(map rename-var args)))
+      (($ $callk k proc args)
+       ($callk k (rename-var proc) ,(map rename-var args)))
+      (($ $branch kt ($ $values (arg)))
+       ($branch (rename-label kt) ($values ((rename-var arg)))))
+      (($ $branch kt ($ $primcall name args))
+       ($branch (rename-label kt) ($primcall name ,(map rename-var args))))
+      (($ $primcall name args)
+       ($primcall name ,(map rename-var args)))
+      (($ $prompt escape? tag handler)
+       ($prompt escape? (rename-var tag) (rename-label handler)))))
+  (rewrite-cont cont
+    (($ $kargs names vars ($ $continue k src exp))
+     ($kargs names (map rename-var vars)
+       ($continue (rename-label k) src ,(rename-exp exp))))
+    (($ $kreceive ($ $arity req () rest) kargs)
+     ($kreceive req rest (rename-label kargs)))))
+
+(define (compute-var-names conts)
+  (persistent-intmap
+   (intmap-fold (lambda (label cont out)
+                  (match cont
+                    (($ $kargs names vars)
+                     (fold (lambda (name var out)
+                             (intmap-add! out var name))
+                           out names vars))
+                    (_ out)))
+                conts empty-intmap)))
+
+(define (peel-loop cps entry body-labels succs preds)
+  (let* ((body-conts (intset-map (lambda (label) (intmap-ref cps label))
+                                 body-labels))
+         (var-names (compute-var-names body-conts))
+         ;; All loop exits branch to this label.
+         (exit (trivial-intset (loop-successors body-labels succs)))
+         ;; The variables that flow out of the loop, as a list.
+         (out-vars (compute-out-vars cps entry body-labels succs exit))
+         (out-names (map (lambda (var) (intmap-ref var-names var)) out-vars))
+         (join-label (fresh-label))
+         (join-cont (build-cont
+                      ($kargs out-names out-vars
+                        ($continue exit #f ($values ())))))
+         (trampoline-cont
+          ;; A $values predecessor for the join, passing the out-vars
+          ;; using their original names.  These will get renamed in
+          ;; both the peeled iteration and the body.
+          (build-cont
+            ($kargs () ()
+              ($continue join-label #f ($values out-vars)))))
+         (fresh-body-labels
+          ;; Fresh labels for the body.
+          (intset-map (lambda (old) (fresh-label)) body-labels))
+         (fresh-body-vars
+          ;; Fresh vars for the body.
+          (intmap-map (lambda (var name) (fresh-var)) var-names))
+         (fresh-body-entry
+          ;; The name of the entry, but in the body.
+          (intmap-ref fresh-body-labels entry))
+         (fresh-peeled-vars
+          ;; Fresh names for variables that flow out of the peeled iteration.
+          (fold1 (lambda (var out) (intmap-add out var (fresh-var)))
+                 out-vars empty-intmap))
+         (peeled-trampoline-label
+          ;; Label for trampoline to pass values out of the peeled
+          ;; iteration.
+          (fresh-label))
+         (peeled-trampoline-cont
+          ;; Trampoline for the peeled iteration, ready to adjoin to
+          ;; CPS.
+          (rename-cont trampoline-cont empty-intmap fresh-peeled-vars))
+         (peeled-labels
+          ;; Exit goes to trampoline, back edges to body.
+          (intmap-add (intmap-add empty-intmap exit peeled-trampoline-label)
+                      entry fresh-body-entry))
+         (peeled-iteration
+          ;; The peeled iteration.
+          (intmap-map (lambda (label cont)
+                        (rename-cont cont peeled-labels fresh-peeled-vars))
+                      body-conts))
+         (body-trampoline-label
+          ;; Label for trampoline to pass values out of the body.
+          (fresh-label))
+         (body-trampoline-cont
+          ;; Trampoline for the body, ready to adjoin to CPS.
+          (rename-cont trampoline-cont empty-intmap fresh-body-vars))
+         (fresh-body
+          ;; The body, renamed.
+          (let ((label-map (intmap-add fresh-body-labels
+                                       exit body-trampoline-label)))
+            (persistent-intmap
+             (intmap-fold
+              (lambda (label new-label out)
+                (intmap-add! out new-label
+                             (rename-cont (intmap-ref body-conts label)
+                                          label-map fresh-body-vars)))
+              fresh-body-labels empty-intmap)))))
+
+    (let* ((cps (intmap-add! cps join-label join-cont))
+           (cps (intmap-add! cps peeled-trampoline-label
+                             peeled-trampoline-cont))
+           (cps (intmap-add! cps body-trampoline-label
+                             body-trampoline-cont))
+           (cps (intmap-fold (lambda (label cont cps)
+                               (intmap-replace! cps label cont))
+                             peeled-iteration cps))
+           (cps (intmap-fold (lambda (label cont cps)
+                               (intmap-add! cps label cont))
+                             fresh-body cps)))
+      cps)))
+
+(define (peel-loops-in-function kfun body cps)
+  (let* ((succs (compute-successors cps kfun))
+         (preds (invert-graph succs)))
+    ;; We can peel if there is one successor to the loop, and if the
+    ;; loop has no nested functions.  (Peeling a nested function would
+    ;; cause exponential code growth.)
+    (define (can-peel? body)
+      (and (trivial-intset (loop-successors body succs))
+           (intset-fold (lambda (label peel?)
+                          (match (intmap-ref cps label)
+                            (($ $kargs _ _ ($ $continue _ _ exp))
+                             (match exp
+                               (($ $fun) #f)
+                               (($ $rec (_ . _)) #f)
+                               (_ peel?)))
+                            (_ peel?)))
+                        body #t)))
+                         
+    (intmap-fold
+     (lambda (id scc cps)
+       (cond
+        ((trivial-intset scc) cps)
+        ((find-entry scc preds)
+         => (lambda (entry)
+              (if (can-peel? scc)
+                  (peel-loop cps entry scc succs preds)
+                  cps)))
+        (else cps)))
+     (compute-strongly-connected-components succs kfun)
+     cps)))
+
+(define (peel-loops cps)
+  (persistent-intmap
+   (with-fresh-name-state cps
+     (intmap-fold peel-loops-in-function
+                  (compute-reachable-functions cps)
+                  cps))))